Imaging method based on two different x-ray spectra

ABSTRACT

The invention relates to a method for the image-producing examination of an object to be examined, especially a patient (P). According to said method, a contrast agent (KM) is first administered to the object to be examined. At least two spatial distributions of x-ray attenuation values are determined, said values respectively representing the local x-ray attenuation coefficients (μ(x,y)) or a quantity (C) which is linearly dependent on the same. The two spatial distributions comprise at least one first attenuation value distribution (μ1(x,y)), the determination thereof being based on a first x-ray spectrum, and one second attenuation value distribution (μ2(x,y)), the determination thereof being based on a second x-ray spectrum which is different to the first x-ray spectrum. By evaluating the two attenuation value distributions, a spatial distribution of at least one predefined atomic number value (Z; Z1, Z2, . . . ) or a spatial distribution (Z(x,y)) of a non-predefined atomic number value in the object to be examined is determined, said spatial distribution containing information about the distribution of the administered contrast agent (KM) in the object to be examined. The spatial atomic number distribution is used to represent the contrast agent (KM) in the image.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP2004/002094 which has anInternational filing date of Mar. 2, 2004, which designated the UnitedStates of America and which claims priority on German Patent Applicationnumber DE 103 11 628.1 filed Mar. 14, 2003, the entire contents of whichare hereby incorporated herein by reference.

FIELD

The invention generally relates to a method for imaging examination ofan examination object that for medical purposes may be, in particular, apatient. The method is suitable in particular for application in thecourse of a tomography method or for application in an imagingexamination device capable of tomography, for example in an X-raycomputed tomography unit.

BACKGROUND

Contrast agents are known, for example, from DE 44 33 564 A1, WO00/16811 or DE 100 02 939 C1.

The result of radiographic methods such as, for example, computedtomography, mammography, angiography, X-ray inspection technology orcomparable methods, is firstly the representation of the attenuation ofan X-ray along its path from the X-ray source to the X-ray detector(projection image). This attenuation is caused by the transradiatedmedia or materials along the beam path, and so the attenuation can alsobe understood as a line integral over the attenuation coefficient of allpixels along the beam path. Particularly in the case of tomographymethods, for example in X-ray computed tomography, so-calledconstruction methods can be used to calculate backward from theprojected attenuation data to the attenuation coefficients (μ) of theindividual pixels, and thus to achieve a substantially more sensitiveexamination than from purely evaluating the projection images.

Instead of the attenuation coefficient, use is generally made for thepurpose of representing the attenuation distribution of a valuenormalized to the attenuation coefficient of water, the so-called CTnumber. This is calculated from an attenuation coefficient μ, currentlydetermined by measurement, and the reference attenuation coefficientμ_(H) ₂ _(O) using the following equation: $\begin{matrix}{{C = {1000 \times {\frac{\mu\quad\mu_{H_{2}O}}{\mu_{H_{2}O}}\lbrack{HU}\rbrack}}},} & (1)\end{matrix}$where the CT number C is in Hounsfield [HU]. A value of C_(H) ₂ _(O)=0HU is used for water, and for air a value C_(L)=−1000 HU.

Since both representations can be transformed into, or are equivalentto, one another, the generally selected term of attenuation value orattenuation coefficient designates below both the attenuationcoefficient μ and the CT value. Furthermore, the terms material andtissue are used exchangeably in the context of the subject of thisdescription of embodiments of the invention. It is assumed that in thecontext of a medically indicated examination a material can be ananatomical tissue, or conversely for material and safety testing atissue is understood to be any desired material of an examinationobject.

Although the informativeness of an image based on the local attenuationcoefficient (μ) is clearly enhanced, problems can nevertheless arisewith interpreting an image in the individual case. Specifically, alocally increased attenuation value can be ascribed either to materialsof higher atomic number such as, for example, calcium in the skeleton oriodine in a contrast agent, or to an increased soft part density suchas, for example, in the case of a pulmonary nodule. The localattenuation coefficient μ at the location {right arrow over (r)} isdependent on the X-ray energy E irradiated into the tissue and/ormaterial and the local tissue and/or material density ρ in accordancewith the following equation: $\begin{matrix}{{\mu = {{\mu\left( {E,\overset{->}{r}} \right)} = {\frac{\mu}{\rho}\left( {E,Z} \right) \times {\rho\left( \overset{->}{r} \right)}}}},} & (2)\end{matrix}$with the energy-dependent and material-dependent mass attenuationcoefficient $\frac{\mu}{\rho}\left( {E,Z} \right)$and the (effective) atomic number Z.

The energy-dependent X-ray absorption of a material as determined by itseffective atomic number Z is therefore superimposed on the X-rayabsorption influenced by the material density ρ. Materials and/or tissueof different chemical and physical composition can therefore exhibitidentical attenuation values in the X-ray image. Conversely, bycontrast, it is not possible to deduce the material composition of anexamination object from the attenuation value of an X-ray picture.

Methods for representing characteristic values of materials are requiredin order to solve this problem. In conjunction with computer-aidedtomography methods, it is known, for example from U.S. Pat. No.4,247,774, to use mutually different X-ray spectra or X-quantum energiesto produce an image.

Such methods are generally denoted as dual-spectrum CT. They utilize theenergy dependence, governed by atomic number, of the attenuationcoefficient μ, that is to say they are based on the effect thatmaterials and tissue of higher atomic number absorb low-energyX-radiation substantially more intensely than do materials and/ortissues of lower atomic number.

By contrast, in the case of higher X-ray energies the attenuation valuesare equal and are largely a function of material density. In the case ofdual-spectrum CT, the differences in the images recorded for differentX-ray tube voltages are then calculated, for example.

Unless otherwise specified, in the context of this description ofembodiments, the term atomic number is used not in the strict sense asreferring to elements. Instead it denotes an effective atomic number ofa tissue, or material, that is calculated from the chemical atomicnumbers and atomic weights of the elements participating in thestructure of the tissue and/or material.

Even more specific statements are arrived at when, in addition, themethod of so-called base material decomposition is applied in the caseof X-ray pictures, for example as described in W. Kalender et al. in“Materialselektive Bildgebung und Dichtemessung mit derZwei-Spektren-Methode, I. Grundlagen und Methodik” [“Material-selectiveimaging and density measurement with the dual-spectrum method, I.fundamentals and methodology”], W. Kalender, W. Bautz, D. Felsenberg, C.Süβ and E. Klotz, Digit. Bilddiagn. 7, 1987, 66-77, Georg Thieme Verlag.

In this method, the X-ray attenuation values of an examination objectare measured with the aid of X-ray beams of lower and higher energy, andthe values obtained are compared with the corresponding reference valuesof two base materials such as, for example, calcium (for skeletalmaterial) and water (for soft part tissues). It is assumed that eachmeasured value can be represented as a linear superposition of themeasured values of the two base materials. For example, a skeletalcomponent and a soft tissue component can be calculated for each elementof the pictorial representation of the examination object from thecomparison with the values of the base materials, the result being atransformation of the original pictures into representations of the twobase materials of skeletal material and soft part tissue.

The base material decomposition and the dual-spectrum method aretherefore suitable for telling apart or distinguishing predefinedanatomical structures or types of material in human and animal tissueshaving a sharply different atomic number.

German Patent Application with the application number 101 43 131discloses a method whose sensitivity and informativeness further exceedsthe base material decomposition and, for example, enables a functionalCT imaging of high informativeness. It permits the calculation of thespatial distribution of the mean density ρ({right arrow over (r)}) andof the effective atomic number Z({right arrow over (r)}) from anevaluation of the spectrally influenced measured data of an X-rayapparatus. Very good contrasts are yielded thereby, for example withreference to the chemical and physical composition of the examinationobject. For example, the representation of the distribution of theatomic number in the tissue permits, inter alia, insight into thebiochemical composition of an object being examined, contrasts based onchemical composition in organs previously represented as of homogeneousdensity, a quantitative determination of body constituents such as, forexample, iodine or the like, and removal of instances of calcificationby segmentation on the basis of the atomic number.

SUMMARY

An object of at least one embodiment of the present invention is tospecify a method that provides new possibilities for improvingsensitivity or for enhancing the informativeness in the case of imagingbased on X-rays that depends on material or atomic number.

An object may be achieved according to at least one embodiment of theinvention by a method for imaging examination of an examination object,in particular a patient, in which

-   a) the examination object is administered a contrast agent,-   b) thereafter, at least two spatial distributions of X-ray    attenuation values are determined, which X-ray attenuation values in    each case represent the local X-ray attenuation coefficient    (μ(x,y)), or a variable (C) linearly dependent thereon, the two    spatial distributions comprising at least:    -   a first attenuation value distribution determined on the basis        of a first X-ray spectrum,    -   a second attenuation value distribution determined on the basis        of a second X-ray spectrum, differing from the first X-ray        spectrum,-   c) two attenuation value distributions are evaluated and a spatial    distribution of one or more predefined atomic number values (Z; Z1,    Z2, . . . ) or a spatial distribution (Z(x,y)) of non-predefined    atomic number values present in the examination object is    determined, which spatial distribution includes information relating    to a distribution of the administered contrast agent in the    examination object, and-   d) the spatial atomic number distribution is used to represent the    contrast agent by imaging.

At least one embodiment of the invention includes the idea that the useof contrast agents can improve functional imaging in X-ray computedtomography. In this context, contrast agents have so far been usedmerely in order, for example, to emphasize blood against its tissuebackground in terms of absorption. No evaluation of a selective naturein relation to material or tissue was made. At least one embodiment ofthe invention furthermore includes, inter alia, the finding that adifference in atomic number measurable by way of two different X-rayspectra can be attained by adding a contrast agent in a tolerable dose.

In the method according to at least one embodiment of the invention, anatomic number value of the contrast agent can be predefined. Inparticular, at least one embodiment of the method can be combined withthe base material decomposition mentioned at the beginning.

The spatial atomic number distribution is preferably determined as atwo- or three-dimensional field, the respective field value being alocal atomic number value at the location represented by the relevantfield. The method can, in particular, be combined with the method of theGerman Patent Application 101 43 131 mentioned at the beginning. Thedisclosure content of this patent application, in particular patentclaims 1 and 7 there, is expressly incorporated into the present patentapplication by reference.

Moreover, in addition to the atomic number distribution a further two-or three-dimensional field is preferably determined whose field valuesrespectively reproduce a local density value.

The use of the spatial atomic number distribution for imaging can beperformed, for example, by displaying an image that displays only datafrom a specific atomic number interval—including the value of the atomicnumber of the contrast agent, for example—or beyond a specific atomicnumber limiting value. It is also possible to convert the measuredatomic number values into a gray-value scale or color scale, in whichcase the value of the atomic number of the contrast agent can beemphasized, or be colored on its own, and to display this scale byimaging it. Such images can be superimposed on or subordinated to aconventional, non-functional attenuation image.

According to a particularly preferred example embodiment, the determinedfield having the atomic number values and the determined field havingthe density values are used for the purpose of calculating a localconcentration or a local quantity of the contrast agent.

In the context of at least one embodiment of the invention, a contrastagent is understood as any agent that leads to an improvement incontrast or intensification in contrast in terms of absorption, that isto say in the X-ray image, after being added to the examination object,in particular after being injected into a patient. This covers bothconventional contrast agents as administered, for example, into theblood vessels for perfusion measurements, in order to emphasize theblood vessels in the image. However, “contrast agents” are alsounderstood as agents that are deposited or built up specifically orselectively; e.g. according to a key-lock principle, only at specificsites in the examination object, and thereby permit an organ function tobe checked.

Such last known device(s)/thing(s)/method(s) can also be so-calledmarkers or tracers. Such a marker is composed, for example, of abiological macromolecule, for example an antibody, a peptide or a sugarmolecule, having a high affinity with the target structure to beexamined, and of a contrast substance—which has additionally doped, forexample—that can be effectively visualized in the X-ray image. Themacromolecule serves, for example, as a so-called “metabolic marker” theeffect of which is that the contrast agent, also denoted overall asmetabolic marker, builds up either exclusively in specific regions, forexample tumors, inflammations or other specific disease sites. Contrastagents are known, for example, from the documents named at thebeginning.

Use is preferably made of a contrast agent having an atomic number ofgreater than 20 or greater than 40. The contrast agent has, inparticular, an atomic number of less than 83 or less than 70.

Particularly advantageous contrast agents include gadolinium, iodine,ytterbium, dysposium, iron and/or bismuth.

According to a further advantageous refinement, the contrast agentincludes an organic compound, in particular an aliphatic hydrocarbon,for example sugar, and/or an amino acid or a peptide.

The contrast agent can be designed for selective deposition at specificsites or in specific tissue parts of the examination object.

In an advantageous refinement, the contrast agent is added in a weightconcentration from the range of 10⁻⁴ to 10⁻⁷, in particular from therange of 10⁻⁵ to 10⁻⁶.

The term “X-ray spectrum” used in the context of embodiments in thisdocument has a more widely cast meaning than only the spectraldistribution of an X-radiation emitted by the X-ray source of theapparatus. On the part of X-ray detectors, as well different spectralcomponents of a radiation having different efficiencies can beconverted, and thereby be weighted differently. The effective spectraldistribution resulting therefrom is likewise denoted in this document asX-ray spectrum.

The two attenuation value distributions need not necessarily be recordedconsecutively as two images with a different tube voltage. Since everyX-ray tube emits a spectrum of a certain width, it is also possiblegiven an appropriate spectrally selective configuration of an associatedreceiving unit, to record the two attenuation value distributionslargely or completely simultaneously. It would be possible for thispurpose, for example, to use filters that can be inserted into the beampath, and/or two separately present X-ray detector arrays.

In particular, a receiving unit is equipped for carrying out the methodwith an X-ray detector array that can select quantum energy.

Particularly with regard to a use of the method described in the GermanPatent Application 101 43 131 mentioned at the beginning, it isespecially advantageous that a first functional dependence of a firstattenuation value of the first attenuation value distribution of densityand atomic number, and at least a second functional dependence of asecond attenuation value, assigned to the first attenuation value, ofthe second attenuation value distribution of density and atomic numberare determined, [and further that the spatial atomic numberdistribution—and optionally a spatial density distribution—is/aredetermined by comparing the first functional dependence with the secondfunctional dependence and, if appropriate, with further functionaldependences].

In this case, the determination of the functional dependence of theattenuation values on density and atomic number for at least one X-rayspectrum is preferably performed by way of reference measurement on acalibration sample or in the form of a simulation on the basis of aphysical model.

According to another preferred refinement, the attenuation valuedistributions are converted into a distribution of the density and adistribution of the atomic number for each of the assigned attenuationvalues of the first attenuation value distribution and of the furtherattenuation value distributions on the basis of the determination of avalue pair for density and atomic number, this being undertaken suchthat the value pair fulfills the specific functional dependences of theX-ray absorption on density and atomic number for the first X-rayspectrum and at least one further X-ray spectrum. If it is therebypossible for density and atomic number to be calculated easily for apixel as interface of the functional dependences of the mutuallyassigned X-ray absorption values of the recorded distributions of theX-ray absorption values.

The first X-ray spectrum advantageously has quantum energy that inrelation to the quantum energy of the second X-ray spectrum favors anX-ray absorption by the photoeffect such that a high resolution isobtained in the determination of the atomic numbers.

In a preferred example embodiment of the present invention, at least oneoperating parameter of the X-ray tube is varied in order to vary anX-ray spectrum for recording the examination object, the X-ray sourceemitting a first X-ray spectrum in a first operating state and emittinga second X-ray spectrum, different from the first X-ray spectrum, in asecond operating state such that a rapid change between two X-rayspectra is rendered possible.

Furthermore, a variation of the detector characteristic is undertaken inorder to vary an X-ray spectrum for recording the examination object,the X-ray detector converting spectral subregions of the X-radiationreceived from the X-ray source into mutually independent electricsignals, and in the process permitting simultaneous recording ofdistributions of the X-ray absorption in the case of different X-rayspectra.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in more detail below with the aid ofexample embodiments, reference being made to the following figures, inwhich:

FIG. 1 shows a flowchart of the method according to the invention inaccordance with an example embodiment,

FIG. 2 shows with the aid of an isoabsorption line the production ofidentical attenuation values μ for materials of different composition,

FIG. 3 a shows an example flowchart of a calculation method fordetermining isoabsorption lines as part of the method in accordance withFIG. 1,

FIG. 3 b shows an example flowchart of the transformation of the X-rayattenuation values to values of material density and atomic number aspart of the method in accordance with FIG. 1, and

FIG. 4 shows two isoabsorption lines of a type of tissue in the case oftwo different X-ray spectra.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

An example embodiment of the method according to the invention isillustrated schematically as a flowchart in FIG. 1. In a first step 1, apatient P is administered a tracer or a contrast agent KM, for exampleby injection into the blood vessels or by swallowing. The patient P isthen examined in an X-ray computed tomography unit 2 (merely indicated),specifically both with evaluation of a first X-ray spectrum S1and—simultaneously or consecutively—with evaluation of a second X-rayspectrum S2 (second step 3) that are selected by appropriately settingthe X-ray computed tomography unit 2.

Carrying out an image reconstruction (third step 5) on the basis of theraw data thus obtained results for each of the X-ray spectra S1, S2 inan attenuation value distribution, for example as a distribution μ₁(x,y)or μ₂(x,y) of the (linear) X-ray attenuation coefficient μ within atransaxial tomogram with coordinates x and y. In a fourth step 7, acomputer-aided transformation of the distributions μ₁(x,y) and μ₂(X,y)of the X-ray attenuation coefficient into an atomic number distributionZ(x,y) takes place. The atomic number distribution Z(x,y) is used in afifth step 8 to display a distribution of the contrast agent KM on amonitor 9.

An effective Z=7.52 results on the assumption of an exemplary injectionof a Gd-based tracer with one Gd atom per 10⁶ water molecules(corresponding to approximately 9 ppm proportion by weight). Comparedwith the water value Z=7.42, this concentration can be verified with theaid of the method in accordance with an example embodiment of FIGS. 3 aand 3 b.

The method in accordance with an example embodiment of FIGS. 3 a and 3 bcan be designed as part of the fourth step 7. Details of this method aredescribed in the German Patent Application with the application number101 43 131, the entire contents of which are incorporate herein byreference.

If the density distribution ρ(x,y) is determined at the same time, theconcentration of the tracer substance can be determined quantitativelygiven a known carrier material, for example resembling water.

The atomic number of the contrast agent KM should have as high adivision as possible from the atomic number of the background material,typically water with Z=7.42.

As an alternative to the method in accordance with FIGS. 3 a and 3 b, itis also possible to use the conventional base material decompositiondescribed at the beginning, in order to determine the spatialdistribution of two or more previously defined atomic number values Z1,Z2, . . . in a transverse tomographic plane (x,y). Such a distributioncan likewise be used to display the contrast agent KM in the image.

Explanations will firstly be given below with regard to explaining themethod in accordance with FIGS. 3 a and 3 b: the isoabsorption line 11of FIG. 2 connects all the value pairs (ρ,Z) to an attenuation value μor C, respectively, that is identical for a defined X-ray spectrum. Theillustration of FIG. 1 makes it plain that information relating to thetype and composition of a tissue or material can be derived in a fashionnot based solely on the attenuation values of an X-ray image.

X radiation is attenuated with a different intensity by differentmaterials and in a fashion dependent on the energy of the X radiation.This is to be ascribed to attenuation mechanisms that act differentlyfor the various materials.

The effective atomic number Z of a specific tissue type, given the titleof atomic number for simplicity in the context of this description, iscalculated from the atomic numbers Z_(i) of the elements participatingin the structure, their atomic weights A_(i) and their localmaterial-equivalent densities ρ_(i) for example as: $\begin{matrix}{Z = \frac{\sum\limits_{i}^{\quad}\quad{\frac{\rho_{i}}{A_{i}}\rho_{i}Z_{i}^{4^{\frac{1}{3}}}}}{\sum\limits_{i}^{\quad}\quad{\frac{\rho_{i}}{A_{i}}\rho_{i}}}} & (3)\end{matrix}$

The result is Z_(ca)=20 for pure calcium, approximately Z_(CaH2)≅16.04for calcium hydride, and approximately Z_(H2O)≅−7.428 for water. Thechemical or else biochemical composition of an object can therefore bedetected very effectively via the atomic number Z.

Calculation of the atomic number distribution and density distributionin an examination region presupposes at least two X-ray pictures of theregion for which the recording geometry is identical but which arecompiled by applying X radiation of different energy. When use is madeof more than two X-ray pictures recorded with the aid of different X-rayenergy, the Z resolution and ρ resolution can be improved, althoughthere is also a consequent increase in the radiation load. Therefore,this possibility does not always apply when a patient is to be examined.

The starting point of the conversion of image data on which attenuationvalue is based into distribution images of the atomic numbers and of thematerial or tissue density is knowledge of the isoabsorption lines foreach X-ray spectrum of an X-ray apparatus.

As already mentioned, in this case an X-ray spectrum is not to beunderstood as the narrowly defined term of spectral distribution of an Xradiation emitted by the X-ray source of the apparatus, but as a widerterm taking account of the different weighting of different spectralregions of the emission spectrum of the X-ray tube on the side of theX-ray detectors. A measured attenuation value is therefore yielded fromthe direct attenuation of the radiation spectrum emitted by the X-raytube, and the spectral efficiency of the X-ray detector used. Bothvalues are installation-specific variables and must be determined eitherdirectly or indirectly by means of the attenuation values of calibrationsamples. They are the basis for calculating the isoabsorption lines.

FIG. 3 a is a sketch of three example methods 300 for modeling and forcalculating a family of isoabsorption lines, specifically a theoreticalmodeling, an experimental determination and a theoretical modeling witha calibration of the curves by way of experimentally determinedparameters.

In principle, there are as many isoabsorption lines to be determined asthe number of attenuation values required for covering the span of X-rayattenuations in the X-ray pictures. There is no need in this case tocalculate an isoabsorption line for each theoretically occurringattenuation value; isoadsorption lines not calculated can be madeavailable if required by interpolation or other suitable averagingmethods.

The basic steps of the theoretical modeling are illustrated in theleft-hand branch of the flowchart of FIG. 3 a. In step S302, the data ofthe X-ray emission spectra S(E) specific to an installation are read inwith the available tube voltages as parameter. For this purpose, thespectral distributions of the X radiation can be measured experimentallyin advance for each individual X-ray installation, or datacharacteristic of a specific type of X-ray source are used.

The determination of the detector apparatus function w(E) is performedin step S303. Here, as well, an accurate measurement of the detectorarrangement can be undertaken in advance, or data characteristic of thedetector type such as, for example, the spectral technical specificationthereof, are used instead.

The calculation of the isoabsorption lines in the form of families ofcurves C_(i)(ρ,Z) or μ_(i)(ρ,Z), respectively, is undertaken in stepS304 on the basis of a physical model that, for each relativecombination of S(E) and w(E)reproduces the X-ray attenuations C_(i) andμ_(i), respectively, for materials with different atomic numbers and inthe case of different material densities.

As an alternative to the theoretical modeling of steps S302 to S304, thefamilies of curves of the isoabsorption lines can also be determinedexperimentally. For this purpose, in step S305 the X-ray attenuations ofcalibration materials of different density and mean atomic number aremeasured in the X-ray apparatus for different relevant combinations ofS(E) and w(E). The measured values form the interpolation points for thefollowing calculation of the families of curves of isoabsorption linesC_(i) to μ_(i), respectively, in step S306.

As a further alternative, the families of curves C_(i) and μ_(i),respectively, modeled on a theoretical basis can be calibrated with theaid of experimentally determined X-ray attenuation values. Theattenuation values required for calibrating the theoretical families ofcurves are measured in step S307, as described above for step S305, withthe aid of suitable calibration materials or phantoms in the X-rayinstallation.

By contrast with the purely theoretical modeling of steps S302 to S304,is not an exact knowledge of the X-ray emission spectra S(E) and w(E)that is a presupposition in this method, but rather parameters of thetheoretical modeling of the families of curves of isoabsorption linesC_(i) and μ_(i), respectively, in step S308. Finally, the calibration ofthe curves in step S309 with the aid of calibration values determinedexperimentally in step S307 defines values for these parameters that arespecific to the X-ray emission spectra and detector apparatus functionsof the X-ray apparatus.

The determination of the isoabsorption lines for the required X-rayattenuation values and combinations of S(E) and w(E) provide thepresuppositions for a transformation of image data, which representattenuation values of the X-radiation upon passage through a tissue,into image data that represent a distribution of the atomic number orthe material density in the relevant tissue.

The three example methods for determining isoabsorption lines can alsobe used in a mixed fashion depending on the task. For example, valueswhich can be determined experimentally only inprecisely or only at greatexpense, or even cannot be determined at all, can be supplemented orhave their accuracy rendered more precise with the aid of theoreticalmodeling. Data uncovered with different methods are then combined instep S310 to form a uniform data record, and are held ready for theimage transformations in step S311.

FIG. 3 b illustrates a transformation method 320 suitable for the methodaccording to at least one embodiment of the invention. At least oneembodiment of the method is based on the families of curves ofisoabsorption lines determined using one of the above-described methods300 and held ready as data record in step S321.

A pixelwise transformation is performed. A transformation of an X-rayattenuation value distribution based on two X-ray images recorded fordifferent X-ray energy spectra but identical recording geometry isassumed below. This is the minimum presupposition for carrying out atransformation according to at least one embodiment of the invention.However, it is also possible to use more than two X-ray pictures inconjunction with more than two different energy distributions of theX-radiation.

The pixels to be transformed are selected in step S322, and theattenuation values C₁ and μ₁, respectively, for this pixel are read inthe following step S323 from the first X-ray image, and C₂ and μ₂,respectively are read from the second X-ray image. The interrogation ofthe X-ray spectrum S₁(E) used for the first X-ray picture, and of thedetector apparatus functions w₁(E) as well as of the correspondingvalues S₂(E) and w₂(E) for the second X-ray image is performed in thesubsequent step S324.

These values form the parameters of a subsequent selection of theisoabsorption lines to be assigned to the respective attenuation values.The spectral distributions S_(i)(E) and w_(i)(E), respectively, can alsobe determined here indirectly, for example via an interrogation of theX-ray voltages U₁ or U₂, respectively, that are used, or of theoperating parameters of the X-ray detectors.

A first curve, which fulfils the conditions C₁ and μ₁, respectively, forthe parameters S₁(E) and w₁(E), and a second curve, which fulfils theconditions C₂ and μ₂, respectively, for the parameters S₂(E) and w₂(E)are selected in step S325 from the data record, held ready in step S321,of isoabsorption lines. An example of a first isoabsorption line 11obtained in such a way, and of a second 41 isoabsorption line isillustrated in FIG. 4.

The point of intersection 42 as intersection set of the two curves 11and 41 is calculated in step S326. The curve intersection 42 can bedetermined, for example, by local linear transformation or by findingthe point of intersection iteratively. Since the two curves 11 and 41represent two different attenuation values for the same pixel andtherefore for an identical subregion of a tissue being examined, twoattenuation values must be caused by the same type of material ortissue. The coordinates (ρ, Z) of the curve intersection point 42therefore reproduce the material density and the atomic number of thetissue subregion to be assigned to the pixel.

Finally, the atomic number value Z thus determined is written into theatomic number distribution as a corresponding pixel value in step S327,while the determined material density value ρ is similarly written intothe density distribution in step S328.

Steps S322 to S328 are repeated for all remaining pixels until aconcluding image output can be performed in step S329. It is possiblefor the step S324 to be skipped in this process, since the spectraldistributions S_(i)(E) and w_(i)(E), respectively, are identical for allpixels of an image.

Example embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A method for imaging examination of an examination object,comprising: administering a contrast agent to the examination object;thereafter determining at least two spatial distributions of X-rayattenuation values, the X-ray attenuation values representing at leastone of a local X-ray attenuation coefficient and variable linearlydependent thereon, the two spatial distributions including at least, afirst attenuation value distribution determined on the basis of a firstX-ray spectrum, a second attenuation value distribution determined onthe basis of a second X-ray spectrum, differing from the first X-rayspectrum; evaluating the at least two attenuation value distributionsand determining at least one of a spatial distribution of one or morepredefined atomic number values and a spatial distribution ofnon-predefined atomic number values present in the examination object,the spatial distribution including information relating to thedistribution of the administered contrast agent in the examinationobject; and using the spatial atomic number distribution to representthe contrast agent by imaging.
 2. The method as claimed in claim 1,wherein an atomic number value of the contrast agent is predefined. 3.The method as claimed in claim 1, wherein the spatial atomic numberdistribution is determined as a two- or three-dimensional field, therespect field value being a local atomic number value at the locationrepresented by the relevant field.
 4. The method as claimed in claim 3,wherein, in addition to the atomic number distribution, a further two-or three-dimensional field is determined whose field values respectivelyreproduce a local density value.
 5. The method as claimed in claim 4,wherein the determined field having the atomic number values and thedetermined field having the density values are used for the purpose ofcalculating a local concentration or a local quantity of the contrastagent.
 6. The method as claimed in claim 1, wherein a contrast agenthaving an atomic number greater than 20 is used.
 7. The method asclaimed in claim 6, wherein a contrast agent having an atomic numbergreater than 40 is used.
 8. The method as claimed in claim 1, wherein acontrast agent having an atomic number less than 83 is used.
 9. Themethod as claimed in claim 1, wherein the contrast agent contains atleast one of gadolinium, iodine, ytterbium, dysposium, iron and bismuth.10. The method as claimed in claim 1, wherein the contrast agentcontains an organic compound.
 11. The method as claimed in claim 1,wherein the contrast agent contains at least one of an amino acid and apeptide.
 12. The method as claimed in claim 1, wherein the contrastagent is designed for selective deposition at least one of at specificsites and in specific tissue parts of the examination object.
 13. Themethod as claimed in claim 1, wherein the contrast agent is added in aweight concentration from the range of 10⁻⁴ to 10⁻⁷.
 14. The method asclaimed in claim 1, wherein a first functional dependence of a firstattenuation value of the first attenuation value distribution of densityand atomic number, and at least a second functional dependence of asecond attenuation value, assigned to the first attenuation value, ofthe second attenuation value distribution of density and atomic numberare determined, and wherein the spatial atomic number distribution—isdetermined by comparing the first functional dependence with the secondfunctional dependence and, if appropriate, with further functionaldependences.
 15. The method as claimed in claim 1, wherein a contrastagent having an atomic number less than 70 is used.
 16. The method asclaimed in claim 1, wherein the contrast agent contains an aliphatichydrocarbon.
 17. The method as claimed in claim 1, wherein the contrastagent is added in a weight concentration from the range of 10⁻⁵ to 10⁻⁶.18. The method as claimed in claim 1, wherein a first functionaldependence of a first attenuation value of the first attenuation valuedistribution of density and atomic number, and at least a secondfunctional dependence of a second attenuation value, assigned to thefirst attenuation value, of the second attenuation value distribution ofdensity and atomic number are determined, and wherein the spatial atomicnumber distribution and a spatial density distribution are determined bycomparing the first functional dependence with the second functionaldependence and, if appropriate, with further functional dependences.